Generally, for an internal combustion engine, there are three kinds of cooling sytems, namely air-cooling liquid-cooling, and a combination type comprising both air-cooling and liquid-cooling.
An air-cooled internal combustion engine has the advantage of being constructed simply and compactly because a delivery pump for cooling liquid and a radiator therefor are unnecessary therein. But, in a different aspect, since a cylinder head thereof tends to be heated excessively due to its relatively high thermal load, an enhancement of output of such an engine is limited.
On the other hand, a liquid-cooled internal combustion engine has an advantage in that an enhancement of output thereof becomes possible owing to its intensive cooling system. However, since the engine requires a delivery pump for cooling liquid and a radiator for such an intensive cooling, it becomes complicated and large in construction (for examle, refer to British Laid Open Patent Publication No. 2127487 A).
A partially liquid-cooled internal combustion engine on the other hand has the advantage that it can be reduced in overall size due to a decreased load on the liquid-cooling apparatus and also due to the accomplishment of a small-scale radiator and a reduced quantity of circulating cooling liquid because a cylinder subjected to a relatively small thermal load is cooled by means of cooling air and at least a portion of a cylinder head subjected to a high thermal load is cooled by means of the cooling liquid.
Conventional partial liquid-cooling type forced air-cooling systems include the following:
Conventional Embodiment-1 (Japanese Utility Model Publication No. 1959-18309).
This cooling system comprises a centrifugal cooling fan disposed at the front side of the engine body of a vertical internal combustion engine so as to rotate about a fore and rear longitudinal axis thereof, a rectangular prism-shaped fan casing arranged so as to cover the cooling fan and the front sides of a cylinder and a cylinder head, a radiator located at an upper corner of the interior within the fan casing, and a cooling air guide casing extended backward from the fan casing throughout the cylinder and the cylinder head. The radiator is adapted to be cooled by the cooling air flow passing through the fan casing, and the cylinder and the cylinder head are adapted to be cooled by the cooling air flow passing through the guide casing.
However, there is the following disadvantage associated with the conventional embodiment -1 that the radiator had to be larger in size, since the cooling air flow delivered from the cooling fan to the radiator is apt to impinge to the upper wall of the fan casing at right angles therewith, a stagnation zone is presented in the space for the location of the radiator. Further, since the cooling air flow passing through the space for the location of the radiator is hindered by the cylinder head disposed at the lower reaches thereof, the air flow resistance is increased. In this way, as the space of the location of the radiator has such a stagnation of the cooling air flow and the flow resistance gets increased, the flow of cooling air supplied to the radiator is decreased. As a result, the heat per unit area radiated from the radiator decreases and the radiator has to be made correspondingly larger in size:
Conventional Embodment-2 (Japanese Utility Model Publication No. 1979-32085).
This cooling system comprises an axial flow fan (a cooling fan) arranged aside at the front position of an engine body of a vertical internal combustion engine, a cooling air guide casing extended in the fore and rear direction from the location of the axial flow fan toward a cylinder and a cylinder head, and a radiator arranged within a space provided laterally outside of the cylinder head as well as to be in the cooling air guide casing in such a manner as extending in the fore and rear direction thereof. The radiator is adapted to be cooled by means of cooling air flowing obliquely backward within the cooling air guide casing, and the cylinder and the cylinder head are adapted to be cooled by means of cooling air flow the course of which is diverted transversely thereof.
However, there are a number of following disadvantages associated with the conventional embodiment-2.
(a) The external size of the internal combustion engine becomes larger.
Since the axial flow fan and the cooling air guide casing project laterally outside of the engine body, the external size of the internal combustion becomes larger.
(b) The transmission arrangement for driving the cooling fan is complicated.
Since the axial flow fan as a cooling fan is disposed aside at the front side of the engine body, a complicated transmission arrangment is required for interlockingly connecting the input shaft of the axial flow fan to the crankshaft in the engine body.
(c) The radiator has to be made larger in size.
Since the cooling air flows obliquely backward relative to the radiator, which is arranged so to extend in the fore and rear direction, the flow of cooling air supplied to the radiator is inevitably uneven along its length. Furthermore, the total quantity of cooling air passing by the radiator is decreased because the cylinder head located at the lower reaches thereof hinders the passing of cooling air flow by acting as a large flow resistance.
In this way, as the flow of cooling air is supplied to the radiator unevenly and in a small quantity, the heat exchanging efficiency of the radiator becomes worse and as a result, a large-sized radiator is necessary owing to the small quantity of radiation of heat per unit area.
(d) The cylinder head and the cylinder are cooled under an unbalanced condition.
Since the flow of cooling air supplied to the radiator is hindered by the radiator located at the upper reaches thereof and, to the contrary a large quantity of cooling air is supplied to the cylinder due to a small flow resistance prevailing at the upper reaches thereof, the cylinder head is apt to be heated excessively by its large thermal load and the cylinder subjected to a comparatively small thermal load is apt to be cooled excessively. Either of these coolings is unbalanced. As a result, the cylinder head tends to have problems and the engine provides bad combustion performance.
Besides the above-mentioned known art, there is also available another cooling system wherein there is provided a cooling air discharge hole in the cooling air guide casing projected laterally outside of the engine body, with a radiator disposed so as to cover the discharge hole, but which has the disadvantage that since the radiator is located outside of the cooling air guide casing projected laterally of the engine body, the external size of the internal combustion engine has to be rather large.